High vacua



Patented Feb. '14, 1939 PATENT OFFICE HIGH VACUA James G. Baxter,Rochester, N. Y., assignor, by mesne assignments, to DistillationProducts,

Inc., Rochester, N. Y., ware a corporation of Dela- No Drawing.Application May 20, 1937,

- Serial No. 143,755

6 Claims. .(01. 230-101 This invention relates to improvements in methodand apparatus for the production of high vacua and more particularly toimproved condensation pump fluids.

It is an object of this invention to provide new and improved fluids foruse as actuating media in a condensation or diffusion pump. Anotherobject is to provide improved methods of producing high vacua. A stillfurther object is to provide improved pumping means. Other objects willbecome apparent from the following description.

In the past mercury was exclusively employed as the actuating fluid in acondensation or diffusion pump. Recently esters of phthalic acid werefound to have superior pumping properties and they have practicallydisplaced the use of mercury for this purpose. Pump fluids must possesstwo properties in particular. Their vapor pressure at the operatingtemperature must be low and the amount of decomposition which theysuffer during use-must be slight. Until the advent of my invention theesters of phthalic acid were the only pure compounds available withthese characteristics.

I have discovered that di-esters of aliphatic dibasic acids containingmore than four carbon atoms have superior properties as condensationpump fluids. It has also been found that the dl-alkyl esters of anyaliphatic dibasic acid have exceptional pumping properties. Di-esterswhich contain at least 12 but not more than 42 carbon atoms arepreferred.

Examples of suitable aliphatic dibasic acids are glutaric, adipic,pimelic, suberic, azelaic and sebacic acids. The esters of sebacic acidare of particular value. These and other aliphatic dibasic acids may beesterifled with any alcohol, but preferably with an aliphatic alcohol.Examples of such alcohols are ethyl, butyl, amyl, hexyl, heptyl,n-octyl, Z-ethylhexyl and nonyl. Mixtures of 2 or more alcohols may beused to yield a mixed ester, if desired. Sebacates having the formula,

largely depend upon the degree of vacua to be produced. Esters having alower number of carbon atoms, as a general rule, have a higher vaporpressure. They are therefore most useful for lower vacuum work, such asin the booster pumps, 5 described in the co-pending Hickman application#130,359 filed March 11, 1937. The higher molecular weight esters have alower vapor pressure and are most useful for the production of thehighest vacua. Esterssuitable for all such 10 purposes will usuallycontain 12 to 42 carbon atoms in the molecule. Those having a boilingpoint of between about and C. at -1 mm. have been found to be best forthe latter purpose.

The esters can be prepared by known esteril5 fication methods. One suchmethod which gives high yields is to mix one molecular portion of theparticular acid selected with anequal weight of toluene and 2-3molecular proportions of the appropriate alcohol. This mixture isstirred un- 20 til a clear solution results and 1% of p-toluene sulfonicacid catalyst, based on the weight of acid is-added. The mixture is thenheated to distillation temperature in a flask provided with .-acondenser. Water formed during the reaction 26 distills with the tolueneand the mixture is condensed, stratified and the toluene returned to thereaction flask. Practically complete esteriiication takes place in about4 hours, while after 8 hours heating, less than .1% free acid remains.30 Toluene and unreacted alcohol are removed by distillation at waterpump pressure and the residual acid, by distillation at about .1 mm.pressure. The esters prepared in this way are odorless and water-whitein color. Any other known 35 method of preparing the esters can, ofcourse, be used. 1

A particular advantage of esters of aliphatic dibasic acids is the easewith which the ester can be produced. It will be noted that the ester-40 iflcation reaction requires much'less time than usual and goespractically to completion .in the short time of 4 hours. For this reasonlittle free acid remains and purification of the reaction mixture to thehigh degree necessary for 45 pumping fluids is considerably simplified.

The esters described have to a remarkable degree,'the desirableproperties of a superior pumpmg. fluid, namely a low vapor pressure atoperating, temperature and high resistances to ther- 60 maldecomposition and oxidation during use.

In addition they have a higher boiling point than the correspondingphthalates and a simpler trapping device can therefore be used. Forinstance 2-ethyl hexyl sebacate, will produce a vacuum ll of 1.9x 10 at25 C. in a 3 compartment condensation pump as compared with 5X for 2-ethyl hexyl phthalate (octoil"). It boils at 175 C. at .1 mm., some 20C. higher than the boiling point of octoil, without any apparentincrease in the rate of decomposition. It is also markedly more stableto air at high temperature, and therefore offers a greater degree ofprotection from the accidental intrusion of air during operation of thepump. The butyl and'amyl sebacates are likewise superior to thecorresponding phthalates and may be expected to replace them where thehighest vacua are not needed. In general, however, the great thermalstability which the esters of aliphatic di-basic acids possess,constitutes their superiority over the corresponding phthalates now inuse as pumping fluids in condensation pumps.

The esters described can be employed in any type oi condensation pump,but preferably are used in condensation pumps designed for organicworking fluids. The construction or design of such pumps is well knownin the art and does not form a part of this invention.

What I claim is:

1. Means for evacuating a closed system which comprises a condensationpump containing as a working fluid a di-alkyl ester of an aliphaticdibasic acid, the ester being one which contains at least 12 but notmore than 42 carbon atoms.

2. Means for evacuating a closed system which comprises a condensationpump containing-as a working fluid a di-ester of an aliphatic dibasicacid, the ester being one which has a boiling point of between about 150and 180 C. at .1 mm.

3. Means for evacuating a closed system which comprises a condensationpump containing a diester of sebacic acid as a working fluid, the esterbeing one which contains at least 12 but not more than 42 carbon atoms.

4. Means for evacuating a closed receptacle which comprises acondensation pump containing a compound having the following formula, asa working fluid:

ROOC CHzCHzCH-aCHzCI-IzCHaCHzCH-zCOOR' in which R and R are hydrocarbonradicles containing 4 to 9 carbon atoms.

5. Means for evacuating a closed'receptacle which comprises acondensation pump containing as a working fluid, di-Z-ethyl hexylsebacate.

6. Means, for evacuating a closed receptacle which comprises acondensation pump containing as a working fluid a di-ester of analiphatic dl-basic acid, the di-basic acid being one which contains atleast four carbon atoms and the diester being one which contains atleast twelve but not more than forty-two carbon atoms.

JAMES G. BAX'I'ER.

